The present invention relates to mass spectrometry, and, more particularly, to mass spectrometers used with gas chromatographs. A major objective of the present invention is to avoid ion source contamination and ion source filament burnout due to ionization of a solvent peak eluting from a gas chromatograph.
The gas chromatograph and the mass spectrometer combine synergistically to provide an instrument, herein referred to as the GC/MS system, of considerable importance in the field of analytical chemistry. A gas chromatograph separates the components of a mixture in solution by volatizing the components of the solution into a carrier gas stream which is passing over a liquid stationary phase. This process takes place in a packed or capillary chromatography column.
A volatized component must be ionized before it can be analyzed by the mass spectrometer. Accordingly, the eluting components are introduced into an ionization chamber of an ion source. Electron impact (EI) ionization is the most common approach to ionization, and is the approach addressed by the present invention.
In EI ionization, a large current is used to heat a filament so electrons are emitted from its surface. The emitted electrons are accelerated toward the ionization chamber by an electric field, with additional directivity being supplied by a reflector and a magnet. The emitted electrons enter the ionization chamber and collide with eluting molecules, which are thus ionized. The ionized molecules are drawn into the analyzer section of the mass spectrometer by the electric field of a "drawout lens". The analyzer section includes a filter which separates ionized molecules according to their mass-to-charge rations.
Mass spectrometers require low operating pressures, and according employ regulated multistage vacuum pump systems which are monitored by high and low vacuum gauges. These pumping systems are capable of maintaining the low pressures required by the mass spectrometer while the effluent from a column is introduced into the mass spectrometer.
A problem can occur in GC/MS systems if a large amount of sample is introduced into the ionization chamber while the filament is on. Generally, the solvent introduced with the sample is much more plentiful than any of the sample components. The elution of the solvent peak from the column can increase the pressure in the ionization chamber significantly. If the filament is on, a surfeit of ionized molecules contaminate the surrounding ion source components and can burn out the filament. Accordingly, it is generally necessary to turn off the ion source filament while a very large peak, such as a solvent peak, is eluting from a column. It is also possible to divert the effluent during solvent elution, but the dead volumes caused by the additional plumbing adversely affect component separation, particularly where capillary columns are used.
There are two classes of approaches to determining when to have the filament off: those based on prediction and those based on detection. Those based on prediction work best when it is known when and how long the solvent peak is going to elute, and when it is known that no peaks of interest will elute before the solvent. In these cases, the filament is left off until a predetermined time has elapsed, during which time the solvent but no peak of interest should have eluted. The filament is then turned on in time to ionize the effluent containing the sample peaks.
This prediction approach is not generally satisfactory. The number of parameters than can be varied during and between chromatographic runs make it difficult to predict solvent peak timing with precision. In addition, there are solvent-mixture systems in which components of interest elute both before and after a solvent peak. Furthermore, it would be desirable to turn off the filament any time a potentially damaging peak arrived at the ionization chamber, whether or not the peak represented the nominal solvent, a component of interest, column bleed, septum or other artifact.
Accordingly, it would be preferable to be able to detect the onset of a solvent or other large peak, and use this detection as a basis for determining when to shut off the filament. For example, the vacuum system of the mass spectrometer generally includes shut-off circuitry associated with its high vacuum gauge. However, the high vacuum gauge is generally too far downstream of the ion source to shut the filament off in time to prevent contamination and potential filament burn-out.
Another approach considered is to use a collector or draw plate current to detect a solvent peak. A collector in the form of an electrode with a positive potential relative to the ionization chamber is often placed just outside the ionization chamber opposite the filament to sweep negative charge out of the chamber. Otherwise, negative charge could accumulate in the chamber and impede emission of electrons from the filament. Elution of a large peak could prevent most electrons from traversing the chamber to reach the collector. A drop in collector current would then be an indication of a solvent peak.
However, the relation between collector current and chamber pressure is complex. For example, the electrons releases by the solvent molecules during ionization can offset the loss of emitted electrons due to ionization. Thus, the drop in collector current is not a sufficiently fast and sensitive indicator of solvent peaks. Similar difficulties confront the use of the draw plate current as a solvent peak indicator.
Heretofore, it has not been practical to detect a solvent peak fast enough to protect the ion source components from contamination and the filament from burn out. Thus, peaks of interest are missed while a filament is off, and damage to the ion source and filament, and loss of a sample run occur as a solvent peak is ionized. What is needed is a reliable method of detecting a solvent peak in time to prevent its ionization.